Interdigitated electrode device

ABSTRACT

An electrode structure has a layer of at least two interdigitated materials, a first material being an electrically conductive material and a second material being an ionically conductive material, the materials residing co-planarly on a membrane in fluid form, at least one of the interdigitated materials forming a feature having an aspect ratio greater than one. A method of forming an electrode structure includes merging flows of an electrically conductive material and a second material in a first direction into a first combined flow, dividing the first combined flow in a second direction to produce at least two separate flows, each separate flow including flows of the electrically conductive material and the second material, merging the two separate flows into a second combined flow, repeating the merging and dividing flow as desired to produce a final combined flow, and depositing the final combined flow as an interdigitated structure in fluid form onto a substrate such that at least one of the materials forms a feature in the structure having an aspect ratio greater than one.

BACKGROUND

Numerous devices such as batteries, fuel cells, electrical interconnectsand others can benefit from tightly spaced interdigitated stripes ofdissimilar materials. The term ‘stripe’ as used here means a line orother shape of material that contains only that material. It does notmix with adjacent stripes of other materials.

Issues arise when trying to produce tightly spaced interdigitatedstripes. In one approach, flow focusing compression produces finefeatures of functional material in paste form. Examples of this approachcan be found in U.S. Pat. Nos. 7,765,949, issued Aug. 3, 2010; and7,799,371, issued Sep. 21, 2010. The approach taken in these patentsrelates to combining materials into ‘co-laminar’ flows, where threelaminar flows of two different materials are brought together to formone flow, but where the two materials do not mix together. This approachsuffices in application where the features are on the order of tens ofmicrons arrayed on a millimeter scale pitch. For example, a solar cellmay have a width of 156 mm and about 80 gridlines, each about 50 micronswide separated by almost 2 mm from a neighboring gridline.

In contrast, the interdigitated structures called for in the design ofelectrodes for energy storage devices may require micron scale featuresinterleaved on the micron scale. For example, a typical cathodestructure may involve interleaved structures that are 5 microns wide and100 microns tall. An electrode structure may be 300 mm wide and 60,000interleaved fingers of dissimilar materials. To dispense these materialsfrom separate nozzles or in from multi-material slot containers would beimpractical.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a fluid flow of two materials into aninterdigitated single flow.

FIG. 2 shows an isometric view of an embodiment of a fluid path.

FIG. 3 shows exploded view of an embodiment of a fluid co-extrusiondevice.

FIG. 4 shows an embodiment of a co-extrusion device and a substrate.

FIG. 5 shows an embodiment of a metal air battery having interdigitatedstructures.

FIGS. 6-10 show embodiments of interdigitated co-extrusions.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In order to attain an interdigitated structure having micron features ona micron scale, it is possible to combine and focus two or more flows,split the combined flow into separate combined flows and then recombineand further focus the flows in repeated stages. This discussion willrefer to the fluidic process that produces interdigitated flows ofdissimilar fluids as ‘fluid folding.’ This discussion may also refer tothe fluidic structure that performs the operations of combining,focusing, splitting, and recombining, etc., as a ‘folding cascade.’

The term ‘focusing’ as used here means the combining of two or moreflows of dissimilar fluids into a combined flow that has a lateralextent across the width of the combined flow at least less than thecombined lateral extent of the flows prior to combination. Typically thewidth of the combined flow after focusing has a lateral extent thatapproximately equals the lateral extent across one of the flows prior tocombination. For example, if a combined flow consists of one ‘stripe’ orfinger each of material A and material B, the combined flow will have alateral extent of measure X. When the flow is split and then recombined,now having two stripes each of materials A and B interleaved, thelateral extent of this flow will have the same lateral extent X of theprevious flow.

FIG. 1 shows a cross sectional diagram of a flow of two materials. Allflows in FIG. 1 are in the direction of the axis that runs perpendicularto the page. All flows are shown in cross section coming out of thepage. Material A, 10, and material B, 12, flow separately from eachother at stage 14. They then combined at stage 16 to form a firstcombined flow. This flow is focused at stage 18. It should be noted thatthe combining and focusing may occur simultaneously or stepwise withinthe cascade.

At stage 20, the combined flow splits into two separate combined flows.Note that the cascade is three dimensional, so the splitting occurs in adirection orthogonal to both the flow direction and the initialcombining and focusing, that is, up and down in the figure.

The two combined flows move separately from each other and are directedto be in lateral proximity at stage 22. At stage 24, the two separatecombined flows are combined into a second combined flow, which is thenfocused at stage 26. This combined flow is then split again at stage 28in a similar or identical fashion as at stage 20, separated at stage 30and then recombined at stage 32. At 34, the combined flow is thenfocused. While this process may repeat as many times as desired, onlylimited by the ability of the materials to remain separated from eachother with no complete mixing when combined, at some point the combinedflow will exit the cascade as a single flow through an exit orifice ornozzle. An advantage of this technique lies in its ability to producematerial features much smaller and more numerous than the fluidicchannels that convey them.

FIG. 2 shows an embodiment of a cascade. A first material enters thecascade through channel 40, and a second material enters the cascadethrough a channel 42. Note that these channels, referred to asseparating channels as they separate or maintain the separation betweenflows, may curve to one side or the other and change levels. The twoflows are combined using a combining channel 44. As discussed above, thecombining channel has a focusing region 46 in which the combined flow iscompressed or focused into a channel having a lateral extentapproximately equal to the lateral extent of either of the individualseparating channels 40 and 42.

The combined flow is then split into two separate combined flows at thejunction of the combining channel 46 and the splitter channels 50 and48. As shown in FIG. 2, the splitter channels split the flows in adirection orthogonal to the direction of the combined flow in thecombining channel 46. In this example, the combined flow is split ‘up’and ‘down’ relative to the combining channel 46. The direction may notbe fully orthogonal, but may be partially orthogonal, such as goingupwards at an angle between straight up and straight ahead. Eachcombined flow in the splitting channels 50 and 48 consists of a stripeor finger of the first material and a stripe or finger of the secondmaterial. As mentioned above, the device is three dimensional and may beformed from layers.

The two separated combined flows are recombined into a second combinedflow by combining channel 52, which also focuses the second combinedflow. The second combined flow in this example consists of fourinterleaved fingers, two each of the first and second materials. Asecond set of splitter channels, 58 and 54 then split the secondcombined flow into two separate combined flows. The structure 58includes another combining channel, forming a third combined flow of 8interleaved fingers, 4 each of the first and second materials.Optionally the structure 58 may also include an exit orifice withchamfered walls to allow the combined flow to exit the cascade as asingle flow.

In operation, looking at FIG. 2, a first material enters the cascade atlayer +1 at 40. The combining layer acts as the reference layer 0. Asecond material then enters the cascade at layer −1 42. These twomaterials combine into combined flow at layer 0, in this case at Ystructure 46. Note that the combined flow consists of two stripes ofmaterial, one each of the first and second materials. Splitter channels48 and 50 then separate the combined flow into two separate combinedflows, each flowing into layers +1 and −1. The layers then recombineinto a second combined flow at 52. Note that the combined flow now has 4stripes of material, 2 each of the first and second materials.

One should note that the structure of FIG. 2 may have abrupt transitionsbetween the layers. This may result in dead volumes in the corners ofthe various transitions where the materials congregate in the cornersinitially and the remaining flow passes by the congregated material.However, over time, and with the device starting and stopping, thecongregated material may harden or otherwise clog the exit orifices. Inaddition these abrupt transitions may induce flow irregularities whichcan lead to substantial or complete mixing of the materials in thestripes. It may then be desirable to have the flow ‘swept,’ meaning thatthe corners are angled or other wise machined, cut or formed, toeliminate the abrupt steps. This is discussed in co-pending application“Oblique Angle Micromachining of Fluidic Structures,” (Attorney DocketNo. 20100587-US-NP-9841-0215).

The splitting and combining processes may continue as long as desiredwithin the constraints of the fluids, which may be pastes, to maintaintheir individual compositions without complete mixing. At each stage ofcombining and focusing, the line count doubles and the width isdecreased for each line by a factor of 2. The cumulative line widthreduction is 2^(n), which is the same for the number of lines. From amanufacturing standpoint, it is useful for the device to be assembledfrom layers fabricated separately and then stacked with an alignmenttolerance. The layers are then clamped or bonded together. FIG. 3 showsan embodiment of such a device.

In this embodiment, the device consists of 9 layers. In this particularexample, bolts, such as that would use bolt hole 63, clamp the devicetogether through corresponding holes on all of the layers. The twomaterials enter from opposite sides of the device. However, this is justan example and no limitation to any particular configuration isintended, nor should any be implied. Further, this particular exampleuses two materials and has 3 cascades repeated 25 times. These allconsist of examples to aid in the understanding of the invention and nolimitation to any particular configuration is intended nor should it beimplied.

A first material enters the device through the sealing plate 63 andenters distribution manifold 61 and the second material enters throughthe opposite facing sealing plate 59 and enters distribution manifold65. Each manifold produces a substantially equalized source of fluidpressure to an array of cascades that will perform the fluid folding.

Optional layers 71 and 75 contain series of ports 60 and 70,respectively. These layers provide one entry point for each of thecascades in the device, and may contribute to the equalization of thepressures of the materials entering the cascades. These layers may alsobe referred to as layers −2 and +2, in order to correspond to the layerreference used above.

On a first fluid folding layer 71, the array of ports 70 conveys a firstfluid from its distribution manifold to an array of separation channels62 on a second fluid folding layer 81. The first fluid is divertedlaterally in a first direction on the second fluid folding layer. On athird fluid folding layer 75, an array of ports 70 conveys a secondfluid from its distribution manifold to an array of separation channels72 on a fourth fluid folding layer 85. On the fourth fluid folding layer85, the second fluid is diverted laterally in a second directionopposite the first direction.

The directions of the separation channels may be flexible. Forconvenience, in this embodiment all of the separation channels on agiven layer all curve in the same direction. Looking at layer 81, forexample, the separation channels in arrays 62, 64 and 66 all divert theflows laterally towards the right side of the drawing. These channelscould go in different directions, or could all go to the left as well.The same holds true for the separating channels on layer 85 in arrays72, 74 and 76.

On a fifth fluid folding layer 95, flows from the second and fourthlayers are combined and focused into a co-laminar flow by the combiningchannels in array 80. The flows then split ‘vertically’ into two flowson the second and fourth folding layers through arrays 64 and 74,respectively. A first combined flow is diverted laterally in the firstdirection on the second fluid folding layer using array 64. The secondcombined flow is diverted laterally into an array of separation channelson the fourth fluid folding layer using array 74.

The flows then return to the fifth fluid folding layer 95, where theycombine and focus into a second combined, co-laminar flow using array82. This process repeats n times, each time doubling the number ofinterdigitated stripes of materials. Downstream of the final stage ofthe splitting and separating, the flows from all of the cascades areoptionally combined together to a common extrusion slot orifice. In theexample provided, there are 3 repetitions of the process resulting in 8interdigitated stripes from each cascade. There are 25 cascades, so theresulting flow will have 200 interdigitated stripes, 100 of eachmaterial.

One should note that while the device shown here has the materialsarranged on opposite sides of the extruding orifice, the materials couldbe introduced on the same side of the orifice

This co-extrusion device of FIG. 3 can be configured and moved relativeto a substrate to deliver the lines of material, as shown in FIG. 4 asdevice 104. The substrate 102 would be positioned in close proximity tothe applicator at a distance that is on the order of 10-1000 microns,referred to as the working distance. The substrate moves relative to thedevice at a speed comparable to the speed with which fluid exits fromthe printhead/applicator 106. The co-extrusion device contains the fluidreservoirs as well as the printhead/applicator 106, as well as controland power circuitry. Optionally the fluid reservoirs may be locatedremotely and fluids delivered to the device as needed through hoses orother plumbing.

In one embodiment, the printhead assembly is configured with componentsthat are chamfered or cut away in such a manner, typically at 45degrees, that the layered assembly may be held a close proximity to thesubstrate at a tilted angle. The tilt of the printhead assembly enablesa feature that the paste exiting the fluid exit orifice forms an obtuseangle (between 90 and 180 degrees) with the deposited paste on thesubstrate. This reduces the bending strain on the extruded paste whichcan aid in the preservation of interdigitated feature fidelity, reducemixing, and increase print speed.

A co-extrusion device such as that shown in FIGS. 2-4 may be used toform devices that benefit from tightly spaced interdigitated stripes ofdissimilar materials including batteries, fuel cells, electricalinterconnects and others. In the case of interconnects, verticallystacked integrated circuits may be interconnected along their edges witha series of metal lines separated by insulating spacers. In the case ofelectrochemical devices such as fuel cells and batteries, theinterdigitated structures can enhance performance in a variety of ways.Air cathodes of metal air batteries could be structured to haveinterdigitated regions of hydrophilic and hydrophobic regions. This willtypically exhibit improved oxygen reduction activity, improving thepower output of the device.

FIG. 5 shows an example of such a device 110. A hydrophobic membrane 114has an electrode 112 residing on it. A separator 116 resides on theelectrode 112. The electrode in this example consists of interleavedfingers of porous, hydrophobic regions 118 and porous hydrophilicelectro-catalyst regions 120. As mentioned above, this can exhibitimproved oxygen reduction activity and improve power output. Further,increasing the surface area of the three-phase boundary where the solidcatalyst particle, liquid electrolyte and gas-phase reactant interact.For expensive catalysts such as platinum, such structures offer thepotential of significant cost reduction.

FIGS. 6-10 show embodiments of interdigitated co-extrusion structuresparticularly useful to battery electrode formation. In FIG. 6, theelectrode 130 consists of two materials. A first material 132 is anelectrode material, such as a cathode or anode active electrode. Thematerial 134 is ionically conductive material, either through solidelectrolyte conduction or through porosity. Alternatively, the regionsof material 134 may be fugitive or sacrificial material removed during alater drying or firing stage in the manufacturing process. In FIG. 6,the thinner, ionically conductive regions traverse the entire thicknessof the electrode layer.

In one embodiment of the formation process for such a feature, theinitial flows prior to folding may consist of two flows of material, oneof 134 and one of 132. Alternatively, there could be three flows priorto folding, one of material 134 surrounded by flows of 132. This can beimportant if the two materials interact differently with the walls ofthe fluidic channels which otherwise could cause lack of symmetry in thecombining, mixing and separation of the flows.

One should note that the deposition of the electrically conductivecathode or anode material and the second material onto the membraneresult in a structure having interdigitated features of differentmaterials in fluid form. Fluid, as that term is used here, means a gel,a paste, a slurry or a suspension. While these structures may progressthrough drying or firing stages, they will initially exist in a fluidform.

Further, at least one of these structures will generally have a highaspect ratio. As used here, the aspect ratio means the ratio of themaximum height to the maximum width of a structure or feature. Lookingat FIG. 6, one can see that the feature 134 in the interdigitatedstructure has a high aspect ratio, its height, running in the directionfrom the top of the page to the bottom, is much larger than its width,running from the left to right across the page. Generally, at least oneof the features formed from one of the interdigitated materials willhave an aspect ratio greater than 1.

In an alternative embodiment, shown in FIG. 7, the ionically conductingregion does not traverse the full thickness of the electrode. This canbe formed in two processes, first forming a blanket coat of cathode oranode material followed by an interdigitated coat of the ionicallyconducting region and the electrode material. A single-step approachwould make use of poly-extrusion where the blanket electrode materialwould be introduced under the ionically conducting material by tailoringthe timing of the introduction of the materials into the printhead.

One must note that the proportions of the materials differ greatly, withthe cathode or anode material 132 having a much greater width than theionically conductive material 134. This may occur in many differentways. For example, the input channels, such as 42 and 40 from FIG. 2,may have different sizes. Alternatively, the flow rate of material putinto the channels could differ, with much more of the material 132entering one of the channels than the material 134.

In FIG. 8, a third material is introduced through the printhead, in thiscase a principally electrically conducting material 140, where the term‘principally’ refers to the material having a higher expression of therelevant characteristic than the other materials. The manipulation ofmaterials in the printhead and the subsequent folding processes can becontrolled to allow these types of structures to be formed. For example,the three materials can be combined in a three way folding operation toform the central layer of the structure and two layer folding can beperformed prior to and subsequent to the application of the centrallayer. This can be performed with three sequential applicators orunified in a single applicator which executes all three foldingoperations. In this embodiment it will be important to align the fluidiclayers so that the features in FIG. 9 are continuous through theextruded structure.

FIG. 9 shows a structure similar to that of FIG. 7, where the material134 was a fugitive material, removed after printing leaving a gap suchas 142. FIG. 10 shows an embodiment similar to FIG. 8, with the fugitivematerial removed leaving gaps 142, and having the principallyelectrically conducting material 140. These gaps could be subsequentlyfilled with electrolyte material such as a liquid electrolyte to makesubstantially ionically conducting regions within the electrodestructure.

These gaps could also subsequently be filled with the opposite of thecathode or anode material and a spacer material which preventselectrical contact of the anode and cathode materials but allows ionictransport between the electrodes, forming the opposing electrodes of anelectrochemical cell such as a battery with alternating cathode andanode regions. Alternatively these gaps could be filled with a secondelectrode material and spacer material forming the opposing electrodesof an electrolytic capacitor or supercapacitor.

One alternative mentioned previously in the discussion involved flowingthree materials. Referring back to FIG. 2, one can see a possibility ofaltering the initial flow. Instead of having only two input channels 40and 42 in to the combining channel 46, one could have three or moreinput channels. An example of this is shown by combination channel 146in FIG. 11. In FIG. 11, the combination channel has 3 input channels,allowing combination of 3 materials. From this point forward in theprocess, the remaining structure would be the same. Instead of foldingtwo-material flows, however, the remaining structures would foldthree-material flows. More than 3 input channels could also be used;this merely provides an example of more than 2 materials.

In this manner, interdigitated structures having micron features on amicron scale can be formed using a co-extrusion device. The co-extrusiondevice may take the form of a printhead, allowing faster formation ofthe structures using printing techniques.

It will be appreciated that several of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations, or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

1. An electrode structure, comprising: a layer of at least twointerdigitated materials, a first material being an electricallyconductive material and a second material being an ionically conductivematerial, the materials residing co-planarly on a membrane in fluidform, at least one of the interdigitated materials forming a featurehaving an aspect ratio greater than one.
 2. The structure of claim 1,wherein the ionically conductive material comprises an electrolyte. 3.The structure of claim 2, wherein the ionically conductive materialcomprises an electrolyte residing in a porous material.
 4. The structureof claim 1, wherein the electrically conductive material is one ofeither cathode material or anode material.
 5. The structure of claim 1,further comprising a third material interdigitated with the first andsecond material.
 6. The structure of claim 1, wherein the secondmaterial extends only partially across a width of the structure from afirst side.
 7. The structure of claim 6, further comprising a thirdmaterial interdigitated with the first and second material and the thirdmaterial extends only partially across the width of the structure from asecond side opposite the first side.
 8. The structure of claim 7,wherein the third material is a second electrically conductive material.9. A method of forming an electrode structure, comprising: merging flowsof an electrically conductive material and a second material in a firstdirection into a first combined flow; dividing the first combined flowin a second direction to produce at least two separate flows, eachseparate flow including flows of the electrically conductive materialand the second material; merging the two separate flows into a secondcombined flow; repeating the merging and dividing flow as desired toproduce a final combined flow; and depositing the final combined flow asan interdigitated structure in fluid form onto a substrate such that atleast one of the materials forms a feature in the structure having anaspect ratio greater than one.
 10. The method of claim 8, furthercomprising: removing the second material from the interdigitatedstructure, leaving voids; and filling the voids with an electrolyte,forming an electrode of a battery from the interdigitated structure. 11.The method of claim 9, wherein the second material is porous, the methodfurther comprising filling the porous material with an electrolyte. 12.The method of claim 9, wherein merging flows of the electricallyconductive material and a second material comprises merging flows of theelectrically conductive material, a second material and a thirdelectrically conductive material.
 13. The method of claim 9, whereinmerging flows comprises merging the flow of the electrically conductivematerial with the second material such that the flow of the electricallyconductive material is much larger than the flow of the second material.